Conventionally, a secondary battery has been used as a power source for a variety of types of a portable device or the like. As the number of charge and discharge cycles of a secondary battery increases, the discharge capacity thereof decreases. Accordingly, charging times of the secondary battery repeatedly used are limited. For this reason, some chargers that charge secondary batteries are configured to determine life spans of the secondary batteries when charging them.
This type of the charger includes a charging device that includes a battery voltage detection circuit configured to detect a battery voltage of a battery pack provided with a secondary battery and battery voltage gradient calculation means configured to calculate a battery voltage gradient based on the detected battery voltage and a battery voltage before a predetermined time (see, e.g., Japanese Patent Application Publication No. 2007-24541 (JP2007-24541A)).
If the voltage of the battery pack before the initiation of charging is equal to or lower than a predetermined voltage value and a battery voltage gradient within a predetermined time period after the initiation of charging is equal to or greater than a first predetermined value, the charging device disclosed in JP2007-24541A determines that the life span of the secondary battery has reached its end. Further, the charging device includes an LED which displays the status of life span deterioration of the battery pack.
Accordingly, the charging device is capable of determining the life span of the secondary battery and notifying a user of the deterioration status of the secondary battery in a simple manner.
Further, a battery deterioration measuring device that measures deterioration of a battery cell, i.e., a secondary battery, is known for including a voltage measurement unit configured to measure a voltage of battery cell, a current measurement unit configured to measure a charging current of the battery cell, and a control unit configured to measure the deterioration rate of the battery cell (see, e.g., Japanese Patent Application Publication No. 2008-123961 (JP2008-123961A)).
If the value of a charging current being supplied during the constant current charging of the battery cell is a first charging current value, the battery deterioration measuring device disclosed in JP2008-123961A obtains a first cell voltage from the voltage value measured by the voltage measurement unit.
Further, if the value of the charging current is changed to a second charging current value smaller than the first charging current value, the battery deterioration measuring device obtains a second cell voltage from the voltage value measured by the voltage measurement unit. Furthermore, in the battery deterioration measuring device, the control unit calculates an internal resistance of the battery cell based on the first cell voltage and the second cell voltage to determine the deterioration rate of the battery cell.
Therefore, according to JP2008-123961A, the batter deterioration measuring device may accurately determine the deterioration of the secondary battery to charge the secondary battery.
Meanwhile, there is a variety of secondary batteries, including a secondary battery whose internal resistance is high in an initial phase in which charging times of the secondary battery is small, and a secondary battery whose internal resistance is low in the initial phase, depending on the types or differences of secondary batteries. Moreover, the voltage of the secondary battery before the initiation of charging considerably varies depending on the remaining capacity of the secondary battery to be charged.
However, since the charging device described in JP2007-24541A uses the voltage before the initiation of charging to determine the life span, the possibility of erroneous determination may be increases because of the type or the remaining capacity of the secondary battery. In addition, the battery deterioration measuring device described in JP2008-123961 needs to accurately measure the charging current and voltage of the secondary battery and to calculate the internal resistance, and accordingly, there is a possibility of measurement errors.